A chemical modelling using UCLCHEM successfully accounts for the properties of HCN/HNC in the gas phase and their evolution in the shock. A very good agreement with the observations is obtained for a steady-state C-shock propagating with velocity \(V_s= 40\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  into a pre-shock medium of density \(\rm n(H)=10^5\hbox{\hspace{1.99997pt}cm$^{-3}$}\)  at a time (\(\sim 2000\hbox{\hspace{1.99997pt}yr}\) ) reasonably consistent with the age of the shock as estimated by [1]}. These results agree with our previous UCLCHEM studies on the chemical composition of L1157-B1 [2]}, [3]}, [4]}, [5]}, [6]}, [5]}, [8]}, [9]}, [10]}.
